The present invention relates to apparatuses for gas-dynamic spraying of powder materials and may be used in machine building and other industries for producing coatings imparting different properties to the surfaces being worked.
Protection of structures, equipment, machinery and mechanisms from corrosion and effects of corrosive media, enhancing characteristics of materials, in particular, production of materials with specified properties and development of relevant resource-saving technologies present scientific and engineering challenges which have major practical significance.
These problems are solved by different methods, among them methods of gas-dynamic spraying of powder coatings which are based on that a powder material is injected in a gas flow and the resulting gas-powder mixture for coating is accelerated to supersonic speed [RU 1618782, RU 1618778]. To increase the powder utilization factor and the quality of spraying, prior to feeding to the supersonic nozzle the gas-powder mixture is heated to the temperature lower than the temperature of melting of powder materials [RU 1773072, WO 91/190161 RU 2038411].
For implementation of these methods the devices comprising a source of compressed gas, a gas heating unit, a powder feeder connected with either a gas heating unit inlet [RU 1603581] or a mixing chamber mounted in front of the supersonic nozzle are used [1674585, WO 91/19016, RU 2010619].
In the first case, the powder material contacts a heating-generating elements of the heating unit resulting in oxidation of powder material particles and their sticking to the element.
In the second case, the powder material does not pass through a gas heating unit, but as in the first case, has to pass through the narrowest portion of the nozzle (throat) which is particularly subject to wear by powder material, especially when solid powders are used (metals, ceramic particles etc.).It is the throat which primarily determines the supersonic nozzle operation and efficiency of the device in general.
Such design is rather awkward, as the mixing chamber is a separate component and the powder feeder should be built hermetic and be operated under high pressure, and therefore, would have a considerable weight.
The mixing chamber between the heating unit and the supersonic nozzle leads to additional heat loss, which means consumption of more power for heating the air and maintaining a prescribed temperature at the supersonic nozzle inlet.
This results in increased risk during operation of the device, as in the case of loss of integrity of hermetic sealing of the powder feeder, the powder will be emitted under high pressure.
The purpose of this invention is to produce an apparatus for gas-dynamic spray coating which would be designed to enhance the stability of operation of the nozzle assembly and prolong its service life, reduce power consumption for maintaining the air temperature at the supersonic nozzle inlet, increase operational safety and reduce apparatus weight.
This is achieved in the apparatus for spraying of powdered material, comprised of a compressed air source connected to the heating unit through a gas conduit, a powder feeder and a supersonic nozzle, by connecting the outlet of the gas heating unit to the supersonic nozzle inlet which is connected, in its supersonic portion, through a conduit to the powder feeder outlet.
This construction for spray coating, as compared with known ones, makes possible increasing the operational stability of the apparatus due to lack of nozzle throat wear. This is achieved as the powder does not pass through the throat and therefore does not induce wear, does not change its characteristics and hence does not affect the performance of the nozzle assembly and the apparatus as a whole.
When using powders of solid metals or ceramics wear of the nozzle walls occurs only in the supersonic portion of the nozzle and does not involve the nozzle throat. As the performance of the supersonic nozzle (in particular, air flow, the Mach number etc.) is determined primarily by the throat area, wear of only the supersonic portion of the nozzle permits a slower change in operational conditions of the nozzle, than when the powder is injected to the chamber in front of the nozzle or to the subsonic portion of the nozzle, thereby ensuring a longer service life of the nozzle.
In this case, an mixing chamber is not necessary, which simplifies the design and reduces the apparatus weight, while connection of the heating unit to the nozzle inlet permits the elimination of heat loss in the mixing chamber.
Coupling of the powder feeder with the supersonic portion of the nozzle permits maintaining a lower pressure in the powder feeder, than that at the nozzle inlet, as the pressure is always lower in the supersonic portion of any Laval (supersonic) nozzle than in the subsonic one. This results in the reduction in powder feeder weight and an increase in operational safety.
The design of the apparatus enables the use of atmospheric, rather than compressed air for transporting the powder from the powder feeder to the nozzle. This reduces the apparatus weight and increases operational safety even more, because in this case the powder feeder should not necessarily be hermetically sealed. For this purpose, at the point of powder injection into the nozzle a pressure below atmospheric should be maintained to provide powder transport by atmospheric air flow.
In order for the powder to be effectively transported by atmospheric air, the cross-sectional areas of the supersonic nozzle at the juncture of the nozzle and the powder-feeder conduit should be related to the throat area per the following relation
Si/Sk greater than =1.3P0+0.8
where Si is the cross-sectional area of the supersonic nozzle at the juncture of the nozzle and the powder feeder conduit,
Sk is the supersonic nozzle throat area,
P0 is the full gas pressure at the supersonic nozzle inlet, expressed in MPa.